Control system for material handling equipment

ABSTRACT

A control system for a hydraulic motor actuated material handling mechanism including a source of fluid power, a directional flow control valve interposed between the source of fluid power and the hydraulic motor, a bridge circuit including a source of electrical energy and having a first variable impedance branch varied by a command control lever and a second variable impedance branch associated with the flow control valve, comparator means connected to the output of the bridge circuit for detecting and amplifying an unbalance in the bridge circuit and means interconnected between the comparator and the directional flow control valve for controlling said valve in a manner corresponding to command movement of the variable impedance branch.

United States Patent Carlson et a1.

[451 Oct. 17,1972

[54] CONTROL SYSTEM FOR MATERIAL HANDLING EQUIPMENT [72] Inventors:Ernest C. Carlson, Schwetzingen, Germany; Arthur J. Howells, Jr.,Western Springs, 111.; Marvin D. Jennings, Naperville, 111.; Donald R.Eastman, Lockport,'lll.; Edward G. Sheets, Plainfield, lll.

[73] Assignee: International Harvester Company,

Chicago, Ill.

[22] Filed: Dec. 22, 1969 [2]] Appl. No.: 887,013

[52] US. Cl. ..2l4/138 R, 214/1 CM, 74/471 XY 1 [51] Int. Cl. ..E02f3/28 [58] Field of Search ..2l4/1 CM, 138; l37/625.46

[56] References Cited UNITED STATES PATENTS 3,214,040 10/1965 Willinger..214/l38 R 2,773,660 12/1956 Rasmussen ..244/85 3,414,136 12/1968 Mooreet a1. ..2l4/1 CM 3,490,340 1/1970 Pedersen ..9l/413 FOREIGN PATENTS ORAPPLlCATlONS 847,656 9/1960 Great Britain ..214/1 32 PrimaryExaminer-Gerald M. Forlenza Assistant Examiner-Jerald M.' ForsbergAttorney-Floyd B. Harman [57] ABSTRACT A control system for a hydraulicmotor actuated material handling mechanism including a source of fluidpower, a directional flow control valve interposed between the source offluid power and the hydraulic motor, a bridge circuit including a sourceof electrical energy and having a first variable impedance branch variedby a command control lever and a second variable impedance branchassociated with the flow control valve, comparator means connected tothe output of the bridge circuit for detecting and amplifying anunbalance in the bridge circuit and means interconnected. between thecomparator and the directional flow control valve for controlling saidvalve in a manner corresponding to command movement of the variableimpedance branch.

11 Claims, 10 Drawing Figures '11 sis AMP PATENTEDBBI 17 m2 SHEET 1 OF 6ERNEST C. CARLSON, MARVIN ARTHUR .r. HowELLS JR DONALD R. EASTMAN BYEDWfRD a. SHEETS W ATT'X PATENTEnnm 11 I972 3.698.580

sum 2 or 6 INVENTORS ERNEST acARLsonl, MARVIN 0. JENNINGS ARTHURJyELLSJR, DONALD R. EAsTMAN PATENTED IIT 17 I973 3.698.580

sum 3 OF 6 INVENTORS ERNEST 0. CA RLSON, MARVIN D. JENNINGS ARTHUR JHOWELLS,JR,DOVALD R EASTMAN BY ED ARD 6. SHEETS Arf'k PATENTEDIIBT 1I912 3,698,580

- SHEET 4 [IF 6 IN VENTORS ERNEST c. CARLSON, MARVIN 0. JENNINGS ARTHURJ? HOWELLS,JR. DONALD R. EASTMAN EDWARD 6. SHEETS BY my.

PATENTEUBBIWIBR V 3.698.580

sum 5 or 6 IN VEN TORS ERNEST C CARLSON, MARVIN D. JENNINGS ARTHUR J.HOWELLS,JR. DONALD R EASTMAN Z1256: SHEETS W I AITT').

PATENTEDum 1 1 m2 3. 6 98.58 0 sum 5 OF 6 *I I I INVENTORS ERNEST c.CARLSON, MARVIN 0. JENNINGS ARTHUR J? HOWELLS,JR. DONALD R. EASTMAN WARDG. SHEETS WI Arr'x CONTROL SYSTEM FOR MATERIAL HANDLING EQUIPMENTBACKGROUND OF THE INVENTION This invention relates to a uniqueelectro-hydraulic or electrical control system for controlling one ormore hydraulic motors. More specifically the invention concerns acontrol system for a material handling mechanism mounted upon a vehiclein which the several linkages and hydraulic motors of the materialhandling device may be easily controlled in a manner to improve cycletimes with a measurable amount of reduced operator fatigue.

DESCRIPTION OF THE PRIOR ART Material handling apparatuses of the priorart as illustrated by a front end loader or a backhoe having one or moremechanical linkages interconnecting the material handling bucket orshovel with a vehicle, each linkage being controlled by a separatehydraulic motor, a separate directional flow control valve which in turnis controlled by a separate lever. For example, backhoes in productiontoday utilize a separate lever controlling each flow control valve whichseparately actuates the boom, rotary actuator, dipper stick and bucket.Thus four separate levers are utilized to perform digging operations forthe current backhoes, and excellent dexterity is required on the part ofthe operator to economically utilize such devices. Similarly, the priorart utilizes purely mechanical linkages for controlling directional flowcontrol valves which actuate the rotary actuator, boom, dipper stick andbucket.

SUMMARY OF THE INVENTION Accordingly, the instant invention relates to anovel and unique electrical control circuit for controlling actuation ofthese various linkages of the article-handling equipment. Preferredembodiments, hereinafter disclosed, incorporate a circuit betweena-command lever and the flow control valve in such a manner to easilycontrol actuation of the flow control valve and is of such simpleconstruction as to permit utilization of a single lever to operateseveral such valves. It is an ob- 'ject of the instant invention toprovide a single lever control system for material-handling mechanismsin which a plurality of directional flow control valves and hydraulicmotors must be controlled. Similarly, it is an object of the instantinvention to further correlate the direction of movement of the singlecontrol lever with directional movement of the linkage to be controlled.Not only is such directional movement correlated to provide a follow-upsystem, but a unique and novel position control system is profferedherein. It is a further object of the instant invention to provide arelatively simple control system which eliminates the dexteriousrequirements of operators of material-handling equipment.

With respect to the control system per se, it is a general object toprovide an economical and easily assembled electrical control circuitfor any hydraulic motor which results in unique operatingcharacteristics of precise and accurate control of a linkage orhydraulic motor as well as the ability to accurately meter fluid througha directional flow control valve. Such a control system may be easilyadapted in fields unrelated to arti- LII cle handling and would includemachine tools and other diverse areas. Finally, it is an object of theinstant invention to provide an electrical control system for ahydraulic motor in which either a followup system or a positioncontrolsystem can be readily realized.

DESCRIPTION OF THE DRAWINGS,

the following specification and claims when taken in conjunction withthe accompanying drawings in which: I FIG. 1 is a perspective view ofthe rear portion of a vehicle having mounted thereon a conventionalbackhoe;

FIG. 2 is a rear elevational view of the directional flow control valveswhich are utilized to direct flow to and from the hydraulic motorsactuating the various linkages of the backhoe;

FIG. 3 is an exploded perspective view of the single handle controlutilized to actuate the directional flow control valves which in turndirect fluid to and from the hydraulic motors;

FIG. 4 is a side elevational view of the rotary actuator of the backhoehaving the boom attached thereto with a schematic disclosure of theelectro-hydraulic control system utilized in controlling the boom of thebackhoe; t

FIG. 5 is a schematic view of an electro-hydraulic circuit forcontrolling the hydraulic motor which in turn operates the boom of thebackhoe;

FIG. 6 is a side elevational view of the rotary actuator and the boom ofthe backhoe attached thereto with a schematic view of an'electro-hydraulic control system which actuates the rotary actuator inturn controlling rotational movement of the backhoe boom;

FIG. 7 is a sc hematic'view of the electro-hydraulic control system ofFIG. 6;

. FIG. 8 is a top view disclosing rotary actuation of the backhoe unitand its correlated movement of the control handle;

FIG. 9 is a perspective view of a mounting element for the control leverof FIG. 3; and

FIG. 10 is a schematic view of an additional control circuit.

DETAIL DESCRIPTION As exemplified in the attached drawings, thepreferred embodiments of our unique invention are incorporated into atractor 10 to which a material-handling apparatus is attached. Thetractor 10 is provided with an operators station 13 having thereon aseat 11 adjacent to which is a single handle control lever 35 operativeto control all of the movements of the associated material-handlingapparatus. In the instant disclosure, this material-handling apparatuscomprises a backhoe having a support stand 12 suitably attached to thetractor by conventional means and being further supported upon theground surface by stabilizer arms 14 (only one of which is here shown),the stabilizer itself being controlled by hydraulic motor 15.Appropriate brackets 16, 16 extending rearwardly from support stand 12carry a rotary hydraulic motor 30 constrained from rotation by suitablemeans and having a shaft 31 extending therethrough for rotatably drivingthe swing mount 17 as fluid energy is directed to the actuator 30. Theswing mount 17 in turn rotatably mounts a boom 18, dipper stick 19 andbucket 20, these linkages and the bucket being controlled in theirmovement by hydraulic rams 21, 22, and 23 respectively.

Disposed underneath the operators station 13 for convenientlycontrolling the movement of the rams 15, the rotary actuator 30, and thehydraulic rams 21, 22 and 23, is a valve bank 62 comprising sixdirectional flow control valves 61. Upon one end of this valve bank ismounted a conventional end cap 63 to which is connected an intakeconduit 67 for delivering fluid from a pump to thevalves. Adjacent theopposite end of the valve bank 62 is another identical end cap 63 towhich is connected a conduit 79 for returning hydraulic fluid to thereservoir. These valves 61 are substantially similar to that disclosedin US. Pat. No.'2,873,762 issued to F. H. Tennis and dated Feb. 17,l959. Further reference may be had to a cross sectional view of thesesame valves in FIGS. 4 and 6 for a clearer understand-' ing thereof.

ELECTRO-I-IYDRAULIC CONTROL CIRCUIT Reference may now be had to FIGS. 4and in which one of the control valves 61 is integrated into the controlcircuit of the instant invention for the hydraulic motor 21 whicheffectuates movement of the boom 18 of the backhoe. (As later explained,such a circuit is associated with each element of the backhoe). A pump Pdelivers fluid from reservoir S to directional control valve 61 througha conduit 67. As disclosed in FIG. 2 such fluid would normally bedelivered to an end cap 63 of valve bank 62, such being omitted fromFIG. 4 for purposes of simplicity. This fluid is then directed into anopen center passage 68 and normally flows directly out of this valve andinto the opposite end cap and back to sump. As more fully disclosed inUS. Pat. No. 2,873,762, a spool 70 is reciprocable within the valve 61whereby the reduced diameters of said spool in conjunction with variouslands and porting will control the direction of fluid flow to and fromthe hydraulic motor 21. Briefly setting out such structural andfunctional characteristics of this well known valve, it will be observedthat if spool 70 is shifted either to the right or to the left and fluidcan no longer flow through the open or low pressure passage 68 since thenormal diameter of the spool precludes such, and fluid is then directedinto a high pressure passage 69 for delivery to the hydraulic motor 21via one of the motor ports 72 or 73.

Thus, assuming the spool is shifted to the right, the low pressure, opencenter feature of the valve is-closed by a normal diameter of the spool,and hydraulic fluid is then urged into the high pressure passage 69,across the reduced diameter of the spool at 81 and out port 72 tocontract the hydraulic ram 21. Fluid is then returned from the oppositeend of the ram 21 via conduit 77, motor port 73, across reduced diameter83 of the spool and out of exhaust port 78 to the sump S through aconduit 79. Reciprocation of the spool in the opposite direction merelyreverses the direction of flow above explained. These passages andportings of the open center valve being quite conventional, no furtherexplanation is deemed necessary other than reference to the aboveidentified patent.

Movement of the spool 70 of the valve 61 is controlled by expansionablechamber device or servo motor 85 and includes a housing 86 connected tothe valve 61 and having a piston element 87 attached to the spoolwhereby any unbalance of fluid energy delivered through a port 88 or aport 89 is effective to cause reciprocation of the piston 87 and thespool 70 to control the direction of fluid flow to hydraulic motor 21.Fluid is supplied to the servo motor from a pump P1 delivering fluidthrough a conduit 113 to solenoid operated valves and 111. Thesesolenoid valves may appropriately be a conventional normally openthree-way valve in which the fluid is normally directed through thevalve to the ports 88 and 89 of servo motor 85 but upon selectiveactuation they will dump both supply fluid and fluid on the appropriateside of piston 87 to sump S whereby the normal pressure on the opposingside of the piston will cause the spool to reciprocate. Anotherpreferable solenoid valve would include the four-way normally opennormally open solenoid valve, Type V955 made and sold by SkinnerPrecision Industries, Inc. of New Britain, Connecticut. This valve isbasically a combination of two three-way valves disclosed above, butincorporated into one housmg.

For controlling actuation of the solenoid valve 110 or 111 andconsequent actuation of spool 70, a bridge circuit is interposed betweena control member or lever 35 and spool 70 of the control valve 61. Asmore clearly depicted in FIG. 5, a bridge circuit is established inwhich two potentiometers 44 and 92 are connected in parallel and avoltage impressed across them. The wiper of potentiometer 44 isrotatably attached to the control lever 35 as hereinafter explained, andthe wiper of potentiometer 92 is rotatably attached by assembly 91 (seeFIG.'2) to the spool 70 of control valve 61.

The wiper leads are then connected to a null detector means orcomparator means 109 which will detect any unbalance or voltagedifferential in the bridge circuit and amplify a signal created by saidunbalance to actuate the appropriate solenoid valve 110 and 111. Thus,upon varying the resistance in potentiometer 44, the comparator means109 will actuate solenoid valve 1 10 or 111 so as to dump fluid from theappropriate chamber on one side of piston 87 and normal pressure on theopposite side will effectuate reciprocation of spool 70, the spoolmovement rotating the wiper of potentiometer 92 such that the bridgecircuit will again be balanced and fluid will continue to be pumpeduntil the potentiometer 44 is returned to neutral. Thus, it should beappreciated that movement of a control lever 35 about a horizontal axiswill produce a corresponding proportional movement of spool 70. Withreference to the components of the electro-hydraulic control system, itis to be noted that the null detector or comparator means 109 is aconventional item and readily obtainable in the market in various forms.An example of other forms of the electrical circuits which may provideexcellent alternatives would include a series connection of two variableresistances in parallel with a series connection of two fixedresistances, the comparator 109 being interconnected between theresistances of each parallel branch. The pump delivering fluid to thesolenoid valves is preferably a low volume and low pressure pump, butmay be the same pump utilized to deliver fluid to the backhoe itself ifacceptable flow rates are provided. The solenoid valves selected for thesystem should preferably have a small flow rate for the intended pumpand pressure since more accurate movement of the spool 70-may beobtained. It should be appreciated that solenoid members acting directlyupon the spool could be utilized as well as other systems includingconventional electro-hydraulic servo valves should the potentiometerreading be taken from the valve in such a manner as to be proportionalto the flow rate through such valve.

Referring back to FIG.'4, the potentiometer 92 is attached to the. spool70 in a simple mechanical manner. For example, a bracket 93 is fixedlyattached to the console 13 of the backhoe apparatus with thepotentiometer secured therein and constrained against rotation. A link94 is then constrained for rotation with the wiper of the potentiometer92, reciprocation of the spool causing rotation of this link through apivotal element 95 so as to vary the impedance of this branch of thebridge circuit.

Each of the directional flow control valves 61 associated with therotary actuator, and the hydraulic motors 21, 22 and 23 may beconveniently provided with such an electrical control circuit.

SINGLE LEVER CONTROL For manipulating the command potentiometer 44 aswell as a command potentiometer for each control circuit associated withmotors 30, 21, 22 and 23, applicants have provided a unique controlmeans whereby movement of said control means produces a similardirectional movement of an appropriate element of a material handlingmechanism. As disclosed in FIG. 3 this control means 35 comprises avertical support 36 which may be appropriately journaled in a housing 57in a manner permitting rotational movement thereof. Attached to thelower end of this vertical support 36'is a gear 38 which upon rotationwill drive a pinion 39 constrained for rotation with the stem of apotentiometer 40, the latter being fixedly supported in housing 57.Thus, as the lever 35 is rotated about a verticalaxis, a varyingresistance in a potentiometer 40 is incurred and since this variablecommand impedance forms one branch of a bridge circuit which is part ofan electrohydraulic circuit associated with a valve 61 controlling flowto actuator 30 and otherwise identical to that disclosed in FIGS. 4 and5, the rotational movement of lever 35 about a vertical axis willcontroll rotational movement of the shaft 31 of rotary actuator 30.

In order to control command impedance 44 and boom 18 and motor 21, aspreviously explained in FIGS. 4 and 5, a yoke 37 is provided on theupper end of vertical support 36, and common apertures 45 connect sameto another yoke member 41 as well as to an extensible arm 46, the latterconnection being made rigid by pin connections extending the adjacentdiagonal apertures (unnumbered). The yoke 41 then extends downwardlyfrom the pivotal connection 53 with the extending arm carrying thepotentiometer 44 constrained for movement therewith. The stem of thepotentiometer 44 carries a pinion 43 which is driven by a rack 42mounted on yoke 37, and thus as the extensible arm 46 is rotated about ahorizontal axis passing through a pivotal connection 53, the stern ofthe poten tiometer 44 is rotated so as to obtain a variable impedance orresistance. This potentiometer 44 appropriately controls movement of theboom 18 as previously discussed in relation to FIGS. 4 and 5 byconnecting same with another bridge circuit element attached to thespool of the directional flow control valve 61 associated with hydraulicmotor 21.

The extensible arm 46 may additionally consist of an outer extensiblemember 47 and an inner member 48 over which the outer member istelescoped. A potentiometer 49 is then fixed by a bracket 50 upon saidouter member, and upon extension and retraction of outer member 47, arack 52 and a gear 51 constrained for rotation with the stern ofpotentiometer 49 creates a variable impedance in said potentiometer.Appropriately, this potentiometer may be integrated in a bridge circuitwith a potentiometer associated with the flow control valve 61 which isconnected to hydraulic motor 22 effectuating movement of the dipperstick 19.

Finally a handle 54 is rotatably secured upon the end of extensiblemember 47, and a bolt member 55 constrained for rotation. by anyconventional means with handle 54 extends through the handle and isjoined to the stern of potentiometer 56 secured to member 47. Again,rotation of the control handle 54 will vary the impedance ofpotentiometer 56, and if this potentiometer is integrated into theelectro-hydraulic circuit of FIGS. 4 and 5 which is further associatedwith motor 23, rolling movement of control handle 54 will cause rollingof the bucket 20.

Thus it should be appreciated that, as constructed, the control means 35permits motions analogous to that of the backhoe itself. For example ifthe operator desires to rotate the backhoe, he merely rotates controllever 35 about its vertical axis causing an unbalance in the bridgecircuit of the associated potentiometers and fluid may be directed tothe hydraulic actuator 30 to rotate the backhoe. Similarly rotationalmotion of the extensible arm 46 about a horizontal axis passing throughthe pivotal connection 53 causes actuation of the hydraulic valvecontrolling the boom and such movement is somewhat analogous to movementof the control means. Similarly extension and retraction of hydraulicram of extensible member 47 will operate the directional flow controlvalve associated with hydraulic motor 22 to extend and retract thedipper stick 19 in an analogous fashion, and finally rotational movementof member 54 will cause similar rotational movement of bucket 20. Thisanalogous and corresponding movement directional of the control memberand the backhoe is a most significant advantage since such is effectiveto reduce cycle time of the handling operation as well as enabling anoperator with little experience to rapidly master the techniques of sucha machine. Thus it should be appreciated that by utilization of acontrol system as disclosed in FIGS. 4 and 5 with a single control lever35 and applying same to each of the control valves 61 located within thevalve bank 62, significant advantages may be obtained.

POSITION CONTROL SYSTEM FIGS. 6 and 7 disclose an additional embodimentof the instant invention in which the directional control valve movementnot only corresponds to movement of the control means 35, but thehydraulic motor and associated linkage itself will accurately correspondto movement of control means 35 whereby a position conthereon apotentiometer 131 whose stem is constrained for rotation with the shaft31 of the motor 30. As schematically disclosed in FIG. 7, thispotentiometer 131 is placed in parallel with the potentiometer 40 butstem rotation is such that a positive increasing voltage ofpotentiometer 40 will result in a decreasing voltage in potentiometer131 upon rotation of actuator 30. Thus, the comparator 109 compares thevoltage difference between potentiometer 92 and potentiometers 40 and130. Accordingly, if the control lever 35 is rotated to the right, thewiper of potentiometer 40 will be moved downwardly as shown in FIG. 7,and the comparator will detect an unbalance and actuate solenoid valve111 to permit spool 70 to be moved downwardly. This downward movementwill deflect the stem of potentiometer 92 as shown by dotted lines andsimultaneously fluid will be directed to the rotary actuator so as torotate same in a clockwise direction. This clockwise movement of therotary actuator in turn controls the wiper of potentiometer 130 toreduce its impedance as the actuator approaches the correspondingposition of control lever 35. Thus, the total resistance seen by thecomparator on one side of the bridge circuit diminishes upon rotation ofthe actuator and the null detector will then gradually actuate solenoidvalve 110 to return the spool to neutral such that potentiometer 92isagain in balance with potentiometers 40 and 130 in parallel. That is,as the difference between 131 and 35 gets smaller, the amount ofhigh-pressure fluid delivered to the hydraulic motor decreasesproportionally because potentiometer 92 is closing off spool 70. Asdisclosed in FIG. 8, an annular movement alpha of control arm 35 willthus result in a corresponding annular movement alpha of the boom 18 anda position control system is effectuated.

As more fully pointed out in FIG. 9, a tubular member 57 may be mountedupon the console 13 to rotatably support control lever 35, apertures 58being provided in the upper end surface of member 57. Appropriately,dowel pins 59 may be inserted into these apertures to limit therotational movement of control means 35 about a vertical axis.Consequently, if the position control system of FIGS. 6 and 7 isutilized to control swing movement of the backhoe, a position controlledsystem may additionally yield a return to dig system. For example, ifdowel pins are placed in the end surface of tubular member 57corresponding to a position of the proposed trench, and corresponding toa position for bucket dump, the outer limits of rotational movement ofthe backhoe will be fixed and by merely rotating the lever 35 betweenthese limits, the rotary motor 30 will always return the boom to theselimits. Under such circumstances thedowel pin may reflect a return todig position and the operator need not concern himself with accuratepositioning of the bucket in a trench.

Although not a part of the instant invention, FIG. 10 discloses acircuit for controlling hydraulic motors which position the stabilizerarms 14, and support the boom structure during the digging operation.Since continuous movement is not desired, the two end valves of bank 62are not provided with potentiometers, and the solenoid valves areoperated by on-off switches, 120 and 121. As opposed to the bridgearrangement previously discussed the remainder of the system, e.g.,valve 61, solenoids, and chamber device 85 remain the same.

SUMMARY In summary, applicants have disclosed a novel,

unique, and most advantageous electrical hydraulicsystem for controllingmovement of associated hydraulic motors by electrically controllingactuation of a flow control valve. A single lever capable of severaldistinct movements may effectively be designed so as to control severaldirectional flow control valves and their associated hydraulic motors.Further, such movements may be correlated with the anticipated movementof the elements to be controlled and a follow-up system is effected inwhich a command signal is followed by a correlated movement of theassociated motor controlled element. In addition to this follow-upsystem, provision has been made for anaccurate position control.Finally, in addition to the aforesaid advantages and results,applicants, by incorporating into their system a solenoid actuated valvewhich is effective to control movement of the flow control valve,accurate metering of fluid through the flow control valve may beeffected and infinite and incremental movements of the associatedhydraulic motors and linkages may result therefrom. It should be readilyappreciated that the system herein disclosed may be employed withnumerous vehicles so as to control many material-handling operations bya single lever. Even in the utilization of continuously rotatinghydraulic motors, the herein disclosed control system may be effectiveto accurately control said motor so as to afford braking and incrementalmovement, such not being heretofore attainable. Naturally, variousutilizations as well as variations of the system herein disclosed willfind application in many fields.

We claim: v

1. In a vehicle having a hydraulic motor actuated material-handlingmechanism, a control system for effecting movement of said motorcomprising:

1. a source of fluid energy on said vehicle,

2. a directional flow control valve including a reciprocating spool andinterconnected between said source and the hydraulic motor forcontrolling flow to and from the hydraulic motor upon displacement ofsaid spool from a normally closed position,

3. command means on said vehicle capable of a directional movementanalogous to a movement of said mechanism,

4. electric circuit means having two variable impedances, one of saidimpedances associated with and responsive to actuation of said spool,the other impedance being associated with and responsive to movement ofsaid command means,

5. detector means interconnectible across said impedances for detectinga voltage difference in the respective electrical signals developedacross said impedances,

6. electro'hydraulic means interconnected between said detector meansand said flow control valve for actuating said flow control means todisplacesaid spool in a direction and for a distance corresponding tothe direction and distance movement of said command means so as tonullify any voltage difference in said impedances and to effect acorresponding directional movement of said material 2. an electricallyoperated hydraulic means for 10 directing fluid flow to and from saidexpansible chamber device for actuating said directional flow controlvalve, said valve means and said expansible chamber device being of sucha capacity as to per mit incremental movement of said directional flowcontrol. valve so as to meter hydraulic fluid to and from the hydraulicmotor. 8. An apparatus as recited in claim 7 in which said electricallyoperated hydraulic means comprises:

handling mechanism at a rate determined by the distance of movement ofsaid command means.

2. An apparatus as recitedin claim 1 in which:

. each of said impedances is a potentiometer, one of said potentiometershaving its resistance varied by the spool of said valve and said otherpotentiometer having its resistance controlled by said command means.

3. An apparatus as recited in claim 2 in which said electro-hydraulicmeans comprises:

1. expansible chamber means on said spool of said l. hydraulic valve.means interconnected between flow control valve for effectingreciprocation said hydraulic circuit and said expansible chamberthereof, and device, said hydraulic valve means being respon- 2.electrically operated hydraulic means supported sive to any unbalance insaid circuit and directing by said vehicle for directing fluid to saidexpansifluid to said expansible chamber device so as to ble chambermeans to actuate said spool. eliminate any unbalance.

9. An apparatus as recited in claim 5 in which said lever means includesmeans for permitting movement thereof analogous to potential movement ofthe motor actuated tool.

10. In a material handling vehicle having an operator control stationand having a moveable linkage coupled at one end portion to said vehicleand supporting a work implement at an opposite end portion, and having asource of hydraulic fluid energy and a control system comprising:

1. actuating means comprising a manually operable actuating memberlocated at said operator control station and capable of positionalmovement functionally analogous to that of said linkage;

4. An apparatus as recited in claim 1 in which said electro-hydraulicmeans comprises:

1. an expansible chamber means operatively associated with saiddirectional flow control valve for controlling actuation thereof,

2. an electrically operated hydraulic means for directing fluid flow toandfrom said expansible chamber device for actuating said directionalflow control valve, said valve directional flow control and saidexpansible chamber device being of such a capacity as to permitincremental movement of said directional flow control valve so as tometer hydraulic fluid to and from the hydraulic motor.

5. In a vehicle having a hydraulic motor actuated tool 2. each of saidimpedances is a potentiometer, the potentiometer associated with theflow control valve having its resistance varied by movement of andhydraulic circuit therefor, acontrol system for con- 2. directional flowcontrol valve means including a trolling movement of said motorcomprising: valve member moveable in opposite directions 1. flow controlvalve in said hydraulic circuit having from a central closed positionwhich precludes a movable member therein for controlling the flow ofhydraulic fluid energyto said motor for direction and flow rate ofhydraulic fluid, permitting flow of said hydraulic fluid energy to 2.lever means on said vehicle capable of a movesaid motor to movesaid'linkage in corresponding ment related to a movement of said tool,opposite directions;

3. circuit means including an electrical source and 3. first sensingmeans coupled to said manually having a first variable impedanceassociated with operable actuating member for developing a first themovable member of the flow control valve, and sensing signalrepresentative of the position of said a second variable impedanceassociated with said actuating member; lever means, 4. second sensingmeans coupled to said moveable 4. comparator means interconnected withsaid cirlinkage for developing a second sensing signal cuit means fordetecting and amplifying an unrepresentative of the position of saidlinkage; balance in the respective electrical signals 5. third sensingmeans coupled to said valve member developed across said variableimpedances, for developing a third sensing signal representative 5.electrical means interconnected between said of the position of saidvalve member;

comparator and said movable member for con- 6. control means responsiveto a differential in amtrolling said movable member in a manner so as toplitude between said first and third sensing signals eliminate saidunbalance in said respective electriattendant movement of said actuatingmember ca] signals. from a neutral position for displacing said valve 6.An apparatus as defined in claim 5 in which: 7 member in a correspondingdirection to permit 1. said flow control valve includes a reciprocatingflow of'hydraulic fluid energy to said motor to inspool therein forcontrolling fluid flow, itiate movement of said linkage in acorresponding direction and responsive to said second sensing signal forgradually restoring said valve member to said closed position as saidlinkage moves to a position corresponding to that of said actuatingmember.

1 1. In a material handling vehicle according to claim 10, and furthercomprising:

stop means associated with said linkage for establish- 5 ing adjustablelimit positions of movement for said linkage and at least one directionof movement thereof.

1. In a vehicle having a hydraulic motor actuated materialhandlingmechanism, a control system for effecting movement of said motorcomprising:
 1. a source of fluid energy on said vehicle,
 2. adirectional flow control valve including a reciprocating spool andinterconnected between said source and the hydraulic motor forcontrolling flow to and from the hydraulic motor upon displacement ofsaid spool from a normally closed position,
 3. command means on saidvehicle capable of a directional movement analogous to a movement ofsaid mechanism,
 4. electric circuit means having two variableimpedances, one of said impedances associated with and responsive toactuation of said spool, the other impedance being associated with andresponsive to movement of said command means,
 5. detector meansinterconnectible across said impedances for detecting a voltagedifference in the respective electrical signals developed across saidimpedances,
 6. electro-hydraulic means interconnected between saiddetector means and said flow control valve for actuating said flowcontrol means to displace said spool in a direction and for a distancecorresponding to the direction and distance movement of said commandmeans so as to nullify any voltage difference in said impedancEs and toeffect a corresponding directional movement of said material handlingmechanism at a rate determined by the distance of movement of saidcommand means.
 2. a directional flow control valve including areciprocating spool and interconnected between said source and thehydraulic motor for controlling flow to and from the hydraulic motorupon displacement of said spool from a normally closed position,
 2. Anapparatus as recited in claim 1 in which:
 2. electrically operatedhydraulic means supported by said vehicle for directing fluid to saidexpansible chamber means to actuate said spool.
 2. lever means on saidvehicle capable of a movement related to a movement of said tool, 2.each of said impedances is a potentiometer, the potentiometer associatedwith the flow control valve having its resistance varied by movement ofthe spool of said valve and the potentiometer of the manually variableimpedance having its resistance varied by movement of the lever means.2. an electrically operated hydraulic means for directing fluid flow toand from said expansible chamber device for actuating said directionalflow control valve, said valve means and said expansible chamber devicebeing of such a capacity as to permit incremental movement of saiddirectional flow control valve so as to meter hydraulic fluid to andfrom the hydraulic motor.
 2. directional flow control valve meansincluding a valve member moveable in opposite directions from a centralclosed position which precludes flow of hydraulic fluid energy to saidmotor for permitting flow of said hydraulic fluid energy to said motorto move said linkage in corresponding opposite directions;
 2. anelectrically operated hydraulic means for directing fluid flow to andfrom said expansible chamber device for actuating said directional flowcontrol valve, said valve directional flow control and said expansiblechamber device being of such a capacity as to permit incrementalmovement of said directional flow control valve so as to meter hydraulicfluid to and from the hydraulic motor.
 3. first sensing means coupled tosaid manually operable actuating member for developing a first sensingsignal representative of the position of said actuating member; 3.circuit means including an electrical source and having a first variableimpedance associated with the movable member of the flow control valve,and a second variable impedance associated with said lever means,
 3. Anapparatus as recited in claim 2 in which said electro-hydraulic meanscomprises:
 3. command means on said vehicle capable of a directionalmovement analogous to a movement of said mechanism,
 4. electric circuitmeans having two variable impedances, one of said impedances associatedwith and responsive to actuation of said spool, the other impedancebeing associated with and responsive to movement of said command means,4. An apparatus as recited in claim 1 in which said electro-hydraulicmeans comprises:
 4. comparator means interconnected with said circuitmeans for detecting and amplifying an unbalance in the respectiveelectrical signals developed across said variable impedances,
 4. secondsensing means coupled to said moveable linkage for developing a secondsensing signal representative of the position of said linkage;
 5. thirdsensing means coupled to said valve member for developing a thirdsensing signal representative of the position of said valve member; 5.In a vehicle having a hydraulic motor actuated tool and hydrauliccircuit therefor, a control system for controlling movement of saidmotor comprising:
 5. electrical means interconnected between saidcomparator and said movable member for controlling said movable memberin a manner so as to eliminate said unbalance in said respectiveelectrical signals.
 5. detector means interconnectible across saidimpedances for detecting a voltage difference in the respectiveelectrical signals developed across said impedances, 6.electro-hydraulic means interconnected between said detector means andsaid flow control valve for actuating said flow control means todisplace said spool in a direction and for a distance corresponding tothe direction and distance movement of said command means so as tonullify any voltage difference in said impedancEs and to effect acorresponding directional movement of said material handling mechanismat a rate determined by the distance of movement of said command means.6. An apparatus as defined in claim 5 in which:
 6. control meansresponsive to a differential in amplitude between said first and thirdsensing signals attendant movement of said actuating member from aneutral position for displacing said valve member in a correspondingdirection to permit flow of hydraulic fluid energy to said motor toinitiate movement of said linkage in a corresponding direction andresponsive to said second sensing signal for gradually restoring saidvalve member to said closed position as said linkage moves to a positioncorresponding to that of said actuating member.
 7. An apparatus asrecited in claim 5 in which said electrical means includes:
 8. Anapparatus as recited in claim 7 in which said electrically operatedhydraulic means comprises:
 9. An apparatus as recited in claim 5 inwhich said lever means includes means for permitting movement thEreofanalogous to potential movement of the motor actuated tool.
 10. In amaterial handling vehicle having an operator control station and havinga moveable linkage coupled at one end portion to said vehicle andsupporting a work implement at an opposite end portion, and having asource of hydraulic fluid energy and a control system comprising:
 11. Ina material handling vehicle according to claim 10, and furthercomprising: stop means associated with said linkage for establishingadjustable limit positions of movement for said linkage and at least onedirection of movement thereof.